Skin and core material selection – Miscibility of polymers

In order to realise a strong skin-to-core bond while keeping manufacturing cycle times short, the aim is to fusion bond skin and core materials, see chapter 2.2. Due to the fact that one specific high performance polymer does not exist and fulfil the requirements for composite prepregs for the skins and foam core structure, a hybrid combination has to be considered.

The aim is therefore to cover the amorphous polymer based foam with semi-crystalline polymer based skins to ensure chemical resistance of the whole structure. This means that two chemically different polymers must be joined or mixed, which is often referred to as blending.

Fusion bonding of two dissimilar polymers is no trivial task, since it requires the miscibility of the polymer pair [115]. Miscibility is here defined as the ability to form an indistinguishable homogenous phase [138]. However, most polymer combinations are not miscible or only miscible by means of a compatibiliser [139].

Krause [140] states that polymer pairs can be compatible even if the polymer pair is not miscible and that the published literature does not deal consistently with the terms ’miscible’

4 Materials and process selection 43 __________________________________________________________________________

or ‘compatible’. In the polymer industry, a compatible polymer pair is often a simple one which shows the desirable characteristics after being mixed as a solid or in solution. Often such polymer pairs are not miscible, since several amorphous phases with different compositions are present, but they are referred to as compatible for the case that there is good adhesion between the phases [140].

There are some indicators for the miscibility of polymers which are listed below, though it is no easy matter to estimate the compatibility of dissimilar polymers [138].

• Similar molecular structure

• Similar molecular weight

• Polarity

In order to investigate the miscibility of polymers, several methods exists [138–141]:

1. Study of the optical behaviour of the polymer blend to determine blurring 2. Measurement of glass transition temperatures to determine polymer phases 3. Nuclear magnetic resonance spectroscopy to investigate the polymer structure 4. Theoretical study to predict the affinity of the polymers to each other by

a. Flurry Huggins theory b. Hansen parameter

In the case of joining skins and foam core, the miscibility of the polymer pair is of great interest, since the aim is to fusion bond skin and foam core with the result of obtaining high bond strength.

The following investigations are performed in the framework of this thesis in order to investigate the miscibility of skin and core polymers:

1. Fusion bonding trials followed by mechanical testing 2. Measurement of glass transition temperatures 3. Hansen parameter determination

4. Literature research for miscibility of the specific polymer pairs

4.1.3.1 Fusion bonding investigations followed by mechanical testing The compatibility of the polymer combinations resulting from suitable available skin and core polymers is investigated by fusion bonding experiments. The different polymers, applied as films, are joined and tested according to a T-peel test (based on DIN EN ISO 11339 [142]) to investigate the bond strength.

Polymer film A and polymer film B are placed in a hot press. In order to strengthen the films for testing, the films are reinforced by a steel mesh. A polyimide (PI) film, covered with release agent (Release-All 45 by Airtech, Luxemburg) is brought between both polymer films along a specific section. The whole stack is heated in the hot press and the polymer films are, if possible, fusion bonded. The PI film functions as a crack initiator with the result that a part of the blend is not joined. The result is a sample with a bonded area and two shanks, which are needed for the T-peel test, see Figure 22.

Combining each suitable skin polymer (PEEK, PPS) with each available core material (PEI, PES, PPSU) leads to the combinations to be investigated as illustrated in Table 7.

Figure 22: Film peel testing

Polymer pairs with PEEK are fusion bonded at 360 °C, 0.2 MPa pressure for 15 minutes, whereas PPS based combinations are joined at 295 °C under 0.2 MPa of pressure with a holding time of 15 minutes. 5 samples for each combination are tested.

Table 7: Polymer blend combinations for compatibility investigations (skin/core material) Core material

Skin material PEI PES PPSU PEEK PPS

PEEK PEEK/PEI PEEK/PES PEEK/PPSU PEEK/PEEK /

PPS PPS/PEI PPS/PES PPS/PPSU / PPS/PPS

Based on the peel test results in Figure 23 and the failure mechanisms, the miscibility of the polymer pairs can be differentiated into three categories: miscible, partially miscible (compatibility) and not miscible. The combination of PEEK/PEI leads to high peel-forces.

During testing, the samples fail within a shank indicating a strong bond. The miscibility is therefore rated as good. PEEK/PEEK and PPS/PPS pairs fail as well in the shanks, indicating the right selection of processing conditions. PEEK/PPSU pairing shows partial miscibility, which might be considered according to Krause [140] as compatibility. Both films can be separated at an average peel force of 35 N.

The results of the PEEK/PES, PPS/PEI, PPS/PES and PPS/PPSU combinations show that these materials are not miscible. With low forces around 2 N the films are easily peeled off.

Microscopic pictures confirm these results. As an example, the samples of the PEEK/PEI combination show no distinct interface, leading to the assumption that molecular interdiffusion has taken place, see Figure 24a. In contrast PEEK/PPSU (Figure 24b) and PEEK/PES (Figure 24c) samples show phase separation and a clear interface, indicating that the material are not miscible but obviously in the case of PEEK/PPSU compatible as shown with the T-Peel test results.

Film A Film B

Film A

Film B

bonded un-bonded

F

F

4 Materials and process selection 45 __________________________________________________________________________

Figure 23: Compatibility of polymers: peel forces

a) b) c)

Figure 24: Interphase of a) PEEK/PEI, b) PEEK/PPSU, c) PEEK/PES 4.1.3.2 Measurement of glass transition temperatures

The most common method to determine the miscibility of a polymer pair is the measurement of glass transition temperatures (Tg) by thermoanalytical methods such as differential scanning calorimetry (DSC) [139]. The existence of a single Tg of the polymer combination indicates mixing of the polymers, whereas the appearance of two Tg’s indicates a two phase immiscible polymer pair [139].

Exemplarily, Figure 25 shows the DSC plot of pure PEEK and PEI as well as the DSC plot of a PEEK/PEI blend (50/50 share). The glass transition temperature of approximately 217 °C can be determined for PEI and 147 °C for PEEK. For the PEEK/PEI blend, only one Tg is

0 20 40 60 80 100 120

PEI PES PPSU PEEK PPS

Peel force [N]

PEEK PPS

Metal mesh

Interface

Metal mesh

Interface Metal mesh

discernible at approximately 207 °C, which indicates that the polymers are miscible. The influence on the crystallisation behaviour of PEEK is discussed later in this chapter.

Figure 25: DSC graphs of PEEK, PEI and a PEEK-PEI blend

In comparison Figure 26 present the thermoanalysis of PEEK, PES, and the PEEK/PES (50/50 share) blend. For the virgin materials, the glass transition temperatures can be determined to be 148 °C for PEEK and 227 °C for PES. The PEEK /PES blend features two Tg’s at 147 °C and 225 °C, indicating two phases.

Figure 26: DSC graphs of PEEK, PES and the PEEK/PES blend

PEI